Vietnam was set Thursday to free the remaining three anti-China protesters in detention since a weekend crackdown on their rally in Hanoi, an official said.

"It is expected that late afternoon today (Thursday) these people will be temporarily released," but their case will be further examined, foreign ministry spokeswoman Nguyen Phuong Nga told reporters.

The three had been investigated for causing public disorder, she said.

They were among almost 50 people who officials say gathered "illegally" on Sunday beside a lake in central Hanoi before being taken away by bus after security agents moved in to disperse the crowd.

Most were quickly released while five others were held until Monday night.

Protesters objected to Beijing's "invasion" of waters in the tense South China Sea where the two nations have a longstanding sovereignty dispute.

On Tuesday the US embassy expressed concern about the detention of people "for what appears to be the peaceful expression of their views".

It said detaining people for exercising the right to peaceful assembly contradicts Vietnam's obligations under the International Covenant on Civil and Political Rights.

The US statement was "inappropriate", Nga said.

"The realisation of citizens' rights of democracy and freedom must comply with regulations and the laws," she said.

Sunday's rally over maritime tensions was the 11th since early June, an unprecedented run in an authoritarian communist country where overtly political demonstrations are rare.

Although people were briefly detained after two earlier demonstrations, subsequent protests were allowed to go ahead until Hanoi authorities on Thursday finally clamped down and issued a stop order.